1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system, such as a copying machine, a printer, or a facsimile device, and more particularly to a laser fusing apparatus that fixes a non-fused image formed on a sheet by a laser beam irradiating unit, and an image forming apparatus provided with the same.
2. Description of the Related Art
An image forming apparatus of an electrophotographic system such as a printer has a fusing apparatus that fixes a toner image, which is formed onto a sheet-like printing medium (hereinafter merely referred to as a sheet, wherein a representative embodiment is a printing sheet) onto a sheet by a thermal fusing. A fusing apparatus of a roller pair system including a fuser roller and a pressure roller has been known as one example of the fusing apparatus described above (for example, see Japanese Unexamined Patent Publication No. 11-38802).
The fuser roller is a roller member having an elastic layer formed on a surface of a hollow core made of a metal such as an aluminum. Generally, a halogen lamp is arranged in the core as a heat source. A temperature control device turns on or off the halogen lamp based upon a signal outputted from a temperature sensor provided on the surface of the fuser roller, thereby controlling the temperature on the surface of the fuser roller to be a target temperature.
The pressure roller is a roller member having a heat-resistant elastic layer made of a silicon rubber formed on a peripheral surface of a core as a cover layer. The pressure roller is in pressed contact with the peripheral surface of the fuser roller, whereby a nip region is formed between the fuser roller and the pressure roller due to an elastic deformation of the heat-resistant elastic layer.
In the configuration described above, the fusing apparatus nips a sheet, having a non-fused toner image formed thereon, at the nip region between the fuser roller and the pressure roller. Due to the rotation of these rollers, the sheet is conveyed, and the toner image on the sheet is fused by a heat from the peripheral surface of the fuser roller to be fixed onto the sheet.
However, in the fusing apparatus of the conventional roller pair system, the fuser roller and the pressure roller are at a room temperature, immediately after a power source is turned on in the morning. The surface temperature of the fuser roller has to increase to a predetermined temperature in order to perform a fusing operation. Therefore, a warm-up time is required. In a stand-by state in which a copying operation is not executed, the surface of the roller has to be maintained to be a predetermined temperature. Accordingly, the roller has to always be heated, even when the copying operation is not carried out, which consumes energy for a heat-retention.
In view of this, a fusing apparatus has been proposed that utilizes a laser power, as a system of efficiently fusing only toner without consuming unnecessary energy (for example, see Japanese Unexamined Patent Publication No. 7-191560).
The apparatus described in Japanese Unexamined Patent Publication No. 7-191560 uses plural lasers in order to heat the toner, whereby a fusing performance, which is insufficient only by a single weak laser, is enhanced. This publication describes that, by virtue of this configuration, an inexpensive laser can be used with low output, resulting in that the whole apparatus can be simplified.
On the other hand, an apparatus has been proposed that includes a unit for preliminarily heating a non-heated recording medium by utilizing an exhaust heat from a sheet heated during the fusing operation in order to aim a reduction in a heat quantity used for the fusing (for example, see Japanese Unexamined Patent Publication No. 2004-20824). This apparatus also includes a cooling unit that derives a heat from the recording medium that has passed through a fusing section. This apparatus also includes a heat transferring unit that transfers a heat, which is derived by the cooling unit, to a recording medium conveying belt that is arranged before the fusing section by a heat pipe and a belt. The recording medium conveying belt preliminarily heats the recording medium that has not yet been subject to the fusing operation and that has the toner image transferred thereon. The heat pipe is used for transferring a large quantity of heat with a small temperature difference (for example, see “Operating Characteristics of the Self-Exciting-Mode-Oscillating-Flow-Heat Pipe” by Takahiro Tanaka, Hiroshi Nomura, and Yasushige Ujiie, on Oct. 29, 2004, at “37th Academic Lecture of College of Industrial Technology” by Research Institute of Industrial Technology, College of Industrial Technology, Nihon University.
However, in the technique described in Japanese Unexamined Patent Publication No. 7-191560, an optical energy conversion efficiency of a laser device, i.e., a ratio of electric power that can optically be outputted as a laser light with respect to electric power applied to the laser device, is low. The laser device generally used has an optical conversion efficiency of 50% or less. Specifically, 50% or more of the electric power is a conversion loss with respect to the applied electric power. The conversion loss described above becomes a heat generated from a laser light source. Therefore, a cooling unit for cooling the generated heat is needed. For example, a unit is needed that cools a heat sink, which is provided at a heat generating portion of the light source, with a unit for blowing wind, such as a fan. Alternatively, a unit is needed, such as a water cooling mechanism, for cooling the heat sink with a water circulating apparatus. Since electric power is separately needed in order to operate the cooling unit, resulting in that the total energy conversion efficiency becomes lower than an optical energy conversion rate for a single laser device. The total energy conversion efficiency is a rate of energy by which a light irradiation can be performed with respect to the total applied electric power necessary for operating the laser, including the electric power applied to the laser device and the electric power applied for cooling.
In the technique described in Japanese Unexamined Patent Publication No. 2004-20824, a heat can be transferred only during the conveyance of a sheet. In the fusing system described in Japanese Unexamined Patent Publication No. 2004-20824, i.e., in the system of heating a sheet with two rollers, the whole sheet is heated with a toner. On the other hand, in the laser fusing system, a laser is irradiated only to a print portion (a portion where the toner is present) for heating this portion. Since the sheet is not directly heated, the heat generated on the sheet is very small. Therefore, in the laser fusing system in Japanese Unexamined Patent Publication No. 2004-20824, the effect of being capable of using the exhaust heat is small.
The apparatus described in Japanese Unexamined Patent Publication No. 2004-20824 includes a first heat pipe serving as a cooling unit for deriving the heat of the sheet, and a second heat pipe that receives the derived heat. The heat of the first heat pipe is transferred to the second heat pipe through a heat transferring unit such as a belt. Further, the heat of the second heat pipe is transferred to the recording medium conveying belt positioned at an entrance to the fusing section. By virtue of the configuration described above, the apparatus is supposed to be complicated. Since there are many components until the heat is transferred to the recording medium, which has not yet been subject to the fusing, from the exhaust heat generated from the recording medium that has already been subject to the fusing, the heat is lost due to a radiation during this period, resulting in that the heat transfer efficiency might be deteriorated.